The invention relates to an enzymatic redox catalytic process to delignify and bleach lignocellulosic materials, and oxidise dyes to remove colour, and more specifically a process involving phenol oxidases and a transition metal complex.
Bleaching of kraft pulp is traditionally performed by sequential reaction with chlorine or chlorine dioxide and sodium hydroxide. Environmental concerns and system closure requirements have opened up new opportunities for biotechnology to replace the use of chlorine or chlorine dioxide in pulp bleaching operations.
Enzyme application for pulp bleaching was first reported by Viikari et al., Proc. Third Intl. Conf. Biotechnol. Pulp Paper Industry, Stockholm, 67-69, 1986. The enzyme xylanase was found to enhance the effect of bleaching chemicals by hydrolyzing the xylan network surrounding lignin. The enzymatic treatment usually results in a 10 to 20% saving of bleaching chemicals. Larger chemical savings can be obtained by using a fungal culture of Trametes (Coriolus) versicolor, but the rate of biodelignification is too slow for commercial application (Paice et al., Tappi J. 72(5): 217-221, 1989, Reid et al., Tappi J. 73 (8): 149-15, 1990). The fungal bleaching effect is accompanied by the secretion of at least two lignin-oxidizing enzymes, laccase and manganese peroxidase (Bourbonnais and Paice, Appl Microbiol Biotechnol 36: 823-827, 1992; Paice et al., Appl. Environ. Microbiol. 59: 260-265, 1993).
Laccase is a multicopper oxidase which reduces oxygen to water and simultaneously performs one-electron oxidation of many aromatic substrates (Reinhammar et al, Boca Raton, Florida: CRC Press, Vol. 3, pp. 1-35, 1984). Laccase alone has a limited effect on pulp bleaching due to its specificity for phenolic subunits in lignin (Higuchi, NG Lewis, MG Paice, eds. Plant cell wall polymers: biogenesis and biodegradation. ACS Symposium Series, 399: 482-502, 1989). It has been reported that the substrate range of laccase can be extended to non-phenolic subunits of lignin by inclusion of a mediator such as 2,2xe2x80x2-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) (Bourbonnais and Paice, FEBS Lett 267: 99-102, 1990). Furthermore, the laccase and ABTS couple was also shown to effectively demethylate and delignify kraft pulp (Bourbonnais and Paice, Appl Microbiol Biotechnol 36: 823-827, 1992);
Over 50% delignification was reported with laccase/ABTS followed by alkaline extraction under conditions of time, temperature and consistency compatible with current bleaching technology (Bourbonnais and Paice, Tappi J 79(6): 199-204, 1996).
Since the initial report with the mediator ABTS, there has been intense research activity to discover a cost-effective laccase/mediator combination, and several nitrogen-containing aromatic compounds are now known to be at least as effective as ABTS. In a series of patent applications, Call describes a process for modifying, breaking down or bleaching lignin, where mediators of the family of N-hydroxy aromatic compounds such as 1-hydroxybenzotriazole (HBT) (WO94/29510), violuric acid (WO 97/36039) and N-hydroxyacetanilide (NHAA) (WO 97/36041) were used. Furthermore, Zing et al. (WO 97/06244) describe the use of nitroso-hydroxy aromatic compounds for enhancing the bleaching activity of laccase for pulp and textile applications. In WO 94/12619, WO 94/12620 and WO 94/12621 Schneider et al. proposed the application of peroxidase enzyme in the presence of hydrogen peroxide and several aromatic compounds to bleach lignin-containing material and textile dye. More recently, Schneider and Pedersen (WO 95/01426) proposed the application of several aromatic compounds to mediate laccase-catalyzed bleaching of textile dyes. Vaheri and Piirainen (International Application WO 92/09741) claim that the oxidizing enzyme laccase can be used in conjunction with manganese ions to reduce consumption of chlorine chemicals when applied in the later stages of bleaching. In their process manganese ions were used to control the redox potential of the pulp within a range between 0.05 to 0.3 V. However, delignification of pulp was not shown under these conditions. In U.S. Pat. No. 5,691,193 (1997) we have described a process for the bleaching of kraft pulp with non-chlorine chemicals where, in a first step, pulp is oxidized either with manganese peroxidase enzyme in the presence of Mn(II) ions and hydrogen peroxide or with laccase enzyme in the presence of 2,2xe2x80x2-azinobis-(3-ethylbenzthiazoline-6-sulfonate) (ABTS) and is followed by an alkaline peroxide bleaching step. Combinations of laccase and aromatic mediators have also been described for other applications such as degradation of polycyclic aromatic hydrocarbons (Johannes et al., Appl. Microbol. Biotechnol 46: 313-317, 1996) and chemical synthesis (Potthast et al., J. Mol. Catal. A: Chem 108: 5-9, 1996).
The. use of transition metal complexes as oxygen or peroxide catalysts has been reported for several applications. In the field of pulping and bleaching, ions of Fe, Mn, Co, Cr, Cu and V with various ligands were studied in oxygen bleaching experiments (Perng et al., Tappi J., 76(10), 139-147 (1993) and Tappi J., 77(11), 119-125 (1994)). From these, only iron complexes (ferricyanide, Fe/2,2xe2x80x2-dipyridyl and ferrate) were found to increase oxygen delignification of pulp. However the oxygen catalyzed reactions were not specific to lignin under the conditions claimed, and significant loss of pulp viscosity occurred. Sodium molybdate and sodium tungstate were proposed by Eckert (Canadian Patent 1,129,161 (1982)) to catalyze acidic H2O2 delignification of krafi pulp. In a more recent study, Kubelka et al. (JPPS, 18(3), J108-114, i992) describe a process consisting of an acidic peroxide stage catalysed by sodium molybdate followed by an alkaline oxygen stage. This process was shown to increase pulp delignification by a factor of about 25% but with an equivalent loss in pulp viscosity. Furthermore, Agnemo (9th ISWPC Proceedings 1997, d2-1 -d2-4) proposed the addition of molybdates to reinforce oxygen and ozone delignification at pH 5.
None of the above described research reports or patent specifications mention the combined use of a transition metal complex with laccase to mediate the catalytic delignification of cellulosic materials and bleaching of pulp. Furthermore, in all applications previously described, there was no mention of the recyclability of the redox mediator.
It is an object of this invention to provide a process for oxidizing a substrate.
It is a particular object of the invention to provide a selective and catalytic process for pulp delignification or bleaching, driven by the enzymatic oxidation of a transition metal complex which can be regenerated for further pulp delignification or bleaching.
In accordance with the invention, there is provided a process for oxidizing a substrate comprising:
a) reacting a substrate to be oxidized with:
i) a phenol oxidase, and
ii) a transition metal complex redox mediator,
b) oxidizing said complex ii) with said oxidase i) from a reduced state to an oxidized state, and
c) oxidizing said substrate with said complex ii) in said oxidized state, said substrate being selected from lignocellulosic pulp, cellulosic pulp, paper dye and textile dye, whereby a lignocellulosic pulp is delignified by said oxidizing, and a cellulosic pulp, paper dye or textile dye is bleached by said oxidizing.
The process of the invention may be applied to the delignification and bleaching of kraft or sulphite pulps employing an oxidative enzyme and a transition metal complex which is a redox mediator and serves as a selective catalytic delignifying or bleaching agent. The principle of the invention is that a transition metal complex is oxidized by a phenol oxidase, for example, a laccase (E.C. 1.10.3.2) or a peroxidase (E.C. 1.11.1.7) and can then diffuse within the lignocellulosic fiber of the pulp to mediate selective catalytic lignin oxidation or bleaching. During this reversible redox process, the complex is continuously regenerated following its reaction with lignin and is made available for further reaction. The complex or mediator is characterized in that it contains a transition metal ion, preferably molybdenum, iron or tungsten, ligated in such a way, with molecules or ions, that the complex has a redox potential between 0.5 and 1.2 V (vs NHE (normal hydrogen electrode)).
Suitable complexes or mediators are those of the general formula 1:
Mm+ (L)n
in which:
M represents a transition metal cation,
m+ represents the charge of the metal cation,
L represents a ligand molecule forming a complex with the metal ion, and
n represents the number of ligands L in the complex.
Suitably M is molybdenum, tungsten or iron; m is an integer of 1 to 6; L is selected from cyano (CN-), bipyridyl(bipy), 1,10-phenanthroline (o-phen), thiocyanato (SCN-), amine (NH3) or carbonyl (CO), and n is an integer of 1 to 8.
This novel process allows the use of small amounts of the complex or mediator and a recycling of the regenerated complex or mediator for further pulp delignification.
The invention can also be applied to bleach dyes from paper or textiles so as to remove or strip colour from paper or textiles.
As employed herein a phenol oxidase refers to an oxidase enzyme having the characteristics or capability of oxidizing phenols, polyphenols, aromatic nitrogen compounds, or transition metal complexes.
The principle of this invention is based on a three step coupled reaction system shown in Equation 1 characterized by I) an oxidase enzyme for example of the family laccase using O2 as electron acceptor or peroxidase with H2O2 where the role of the enzyme is to oxidize a mediator, II) a transition metal redox mediator cycling between its reduced state (Mm+) to its oxidized state (M(m+1)+) and III) the lignin polymer in the pulp fibre or a dye which is oxidized and solubilized following its reaction with the mediator. 
Equation 1: representation of the delignifying process
The present invention can be applied to delignify and bleach hardwood and softwood kraft or sulfite pulps produced by a batch or continuous process including lower-lignin content pulps such as those produced by modified continuous cooking or by oxygen delignification or by xylanase delignification.
The pulp is suitably washed with water and acidified for example by adding sulfuric acid or by adding gases such as carbon dioxide or sulfur dioxide to pH between 3 and 6. The acidified pulp at a consistency of between 2 and 20% is mixed with a solution of the mediator containing the phenol oxidase, for example, laccase with oxygen, or peroxidase with hydrogen peroxide. In a particular embodiment illustrating the invention, the pulp is placed in a vessel at a temperature of between 25xc2x0 C. and 70xc2x0 C. for a period of time between 30 minutes and 240 minutes. For laccase treatment, the reaction proceeds under oxygen atmosphere preferably with a pressure of between 100 kPa to 500 kPa.
The present invention can also be applied under essentially the same conditions to dye bleaching in order to modify or remove colour from paper and textiles.
At the end of the treatment, the pulp is pressed, and the extracted liquor may be kept for further batchs of pulp delignification. The recycled mediator can be used to delignify a new batch of pulp or used to delignify the same pulp in a second cycle following an alkaline extraction. With or without further washing, the pulp is then extracted, for example, with 2% sodium hydroxide at 70xc2x0 C. for 90 minutes. The alkaline extraction (E) may be done under an atmosphere of oxygen (Eo) and may contain hydrogen peroxide (Ep). The resulting pulp can be bleached in accordance with any of the well-known conventional bleaching sequences such as ECF with chlorine dioxide steps or TCF with oxygen based chemicals (i.e. O3 or H2O2).
The amount of mediator used is specified in % (g per 100 g of oven dried pulp). Suitable amounts of the mediator can be readily determined by experiment. In particular, trial amounts of mediator of 0.05% to 1% have provided good results. By way of example, suitable mediators or complexes include potassium octacyanomolybdate (K4Mo(CN)8. 2H2O) which may be prepared according to Furman and Miller (Inorg. Synth. 3, 160-163, 1950); potassium octacyanotungstate (K4W(CN)8. 2H2O) which may be synthesized by the method described by Heintz (Inorg. Synth. 7, 142-146, 1963); iron tris-4,4xe2x80x2dimethyl-bipydine [FeII(4,4xe2x80x2dmbpyr)3] which may be prepared by mixing FeCl2 with three molar equivalent of 4,4xe2x80x2dimethyl-2,2xe2x80x2-dipyridyl; and iron tris-o-phenanthroline [FeII(o-phen)3] which is available commercially.
The amount of oxidase enzyme added is specified in units of activity where one unit refers to the amount of laccase or peroxidase needed to oxidize one xcexcmole of ABTS per minute in the presence of sodium acetate buffer (0.1 M, pH 5.0) at room temperature.
Suitable amounts of the enzyme can be readily determined by experiment. In particular, amounts of enzyme of at least 1 U/g and preferably 1 to 20 U/g of pulp have provided goods results.
Laccase is preferably from a fungal source such as that available from commercial suppliers, while peroxidase can be derived from plant or fungal source and is also available from commercial suppliers.